Side-emitting collimator

[0001] 1. Field of the Invention

[0002] The present invention relates generally to lenses for distributing light from a light source and more particularly to a lens for changing the effective direction of light emission for a light source.

[0003] 2. Description of the Related Art

[0004] The use of LED's in warning and signaling lights is well known. Older models of LED's produced limited quantities of light over a relatively narrow viewing angle centered around an optical axis of the LED. These LED's were typically massed in compact arrays to fill the given illuminated area and provide the necessary light output. Modern LED's produce significantly greater luminous flux per component, permitting fewer LED's to produce the luminous flux required for many warning and signaling applications. It is known to arrange a small number of high-output LED's in a light fixture and provide each high-output LED with an internally reflecting collimating lens. The collimating lens gathers light from the LED into a collimated beam centered on the LED optical axis. Such an arrangement typically does not fill the light fixture, resulting in an undesirable appearance consisting of bright spots arranged against an unlit background. Light-spreading optical features on the outside lens/cover are sometimes employed to improve the appearance of the light fixture.

[0005] FIG. 1 illustrates a prior art collimator of a configuration frequently employed in conjunction with LED light sources. Light from an LED positioned in a cavity defined by the collimator is organized into a collimated beam aligned with the optical axis of the LED. The known internally reflecting collimator for an LED is a molded solid of light transmissive plastic such as acrylic. The radial periphery of the collimator is defined by an aspheric internal reflecting surface flaring upwardly and outwardly to a substantially planar light emission surface. The bottom of the collimator includes a cavity centered over the LED optical axis. The cavity is defined by a substantially cylindrical side-wall and an aspheric upper surface. The aspheric upper surface is configured to refract light emitted at small angles relative to the LED optical axis to a direction parallel with the LED optical axis. The shape of the aspheric upper surface is calculated from the refractive properties of the air/acrylic interface, the position of the LED point of light emission relative to the surface and the desired direction of light emission, e.g., parallel to the LED optical axis. The mathematical relationship between the angle of incidence of a light ray to a surface and the angle of the refracted ray to the surface is governed by Snell's Law: “The refracted ray lies in the plane of incidence, and the sine of the angle of refraction bears a constant ratio to the sine of the angle of incidence.” (sin θ/sin θ′=contant, where θ is the angle of incidence and θ′ is the angle of refraction)

[0006] To allow the collimator of FIG. 1 to be easily extracted from a mold, the substantially cylindrical side-wall of the cavity is typically canted at an angle of between 1° and 3° relative to the central axis of the collimator. The cavity is narrower at the top where the side-wall joins the aspheric upper surface than at the bottom of the collimator. For any particular point on the substantially cylindrical side-wall, the path of light refracted into the collimator can be calculated using Snell's law. The shape of the peripheral aspheric internal reflecting surface is calculated from the path of light refracted by the substantially cylindrical side-wall surface and the desired direction of light emission, e.g., parallel to the LED optical axis. The resulting aspheric internal reflecting surface redirects light incident upon it in a direction parallel to the optical axis of the LED.

[0007] The result is that substantially all of the light emitted from the LED is redirected parallel to the optical axis of the LED to form a collimated beam. This arrangement efficiently gathers light from the LED and redirects that light into a direction of intended light emission. Unless the light is somehow spread, the light from each LED appears to the viewer as a bright spot the size and shape of the collimator.

[0008] Briefly stated, a side-emitting collimator according to the present invention comprises a pair of collimator halves configured to meet over an LED to form a cavity defined by refracting surfaces. The upper boundary of each collimator half is defined by a surface configured to internally reflect light refracted into the collimator lens by one of the refracting surfaces. Each half of the side-emitting collimator redirects light from the light source in a direction substantially perpendicular to the optical axis of the light source. The side-emitting collimator redirects the light from an LED into diametrically opposed collimated beams perpendicular to the optical axis of the light source.

[0009] Each half of the side-emitting collimator includes an aspheric upper internal reflecting surface whose shape is calculated from the path of light refracted by the corresponding internal refracting surface. As used in this application, the term “aspheric” means “not spherical”. The illustrated aspheric surfaces are created by rotating a non-circular curve about an axis of symmetry. These surfaces can be described as “rotationally symmetric” about the axis of symmetry. Each half of the side-emitting collimator defines one half of a cavity that receives the light source. The upper surface of the cavity includes two adjoining substantially cylindrical surfaces that meet over the optical axis of the LED light source. Each end of the cavity is defined by one half of an aspheric surface. The partial-cylindrical surfaces are positioned to refract light from the light source onto the upper internal reflecting surface. The partial aspheric refracting surface redirects light from the light source into a direction of light emission parallel to light reflecting from the partial aspheric upper internal reflecting surface.

[0010] These surface shapes and relationships are an example of surfaces that efficiently gather light from an LED and re-direct that light into a pair of opposed collimated beams perpendicular to the optical axis of the LED. The invention should not be construed as being limited to the particular disclosed surface shapes.

[0021] Exemplary embodiments of the side-emitting collimator will now be described with reference to FIGS. 2–6. FIG. 2 shows an exemplary side emitting collimator 10 in conjunction with an LED light source 20. The side-emitting collimator 10 organizes light from the LED 20 into a pair of opposed collimated beams 11 perpendicular to the optical axis A_O of the LED 20. The LED 20 includes a base 26 above a heat-transmissive slug 24. A lens 28 extends upwardly from the base 26. The LED lens 28 surrounds a point of light emission 22. The point of light emission 22 in an LED 20 is defined by a semi-conductor chip (not shown) which emits light when energized by an electrical current. The illustrated LED lens 28 is of the high-dome or “lambertian” shape. An LED with this lens shape emits light in a half dome surrounding the optical axis A_O of the LED. The side-emitting collimator 10 is constructed about a focal point F that preferably coincides with the point of light emission 22 of the LED 20. The left and right portions 10a, 10b of the side-emitting collimator 10 are identical. Each half 10a, 10b of the side-emitting collimator is a solid molded from optical-grade plastic, such as acrylic. Other forming methods and materials may also be compatible with the present invention.

[0022] FIGS. 3 and 4 are sectional views of light assemblies employing side-emitting collimators 10 according to aspects of the present invention. In the assembly of FIG. 3, angled reflecting surfaces 72 redirect the collimated beams after they emerge from the substantially planar light emission surfaces 18 of the side-emitting collimator 10. The angled reflecting surfaces 72 are laterally spaced apart and configured to each intercept a portion of the collimated beam from each side-emitting collimator 10. This arrangement distributes light from each LED 20 over a large area in a direction of light emission B for the light assembly. The reflecting surfaces 72 are also shown in FIGS. 8 and 9.

[0023] FIG. 4 is an alternative embodiment where the side-emitting collimator 10 is incorporated into a larger internally reflecting lens. The reflecting surfaces 72 shown in FIG. 4 are internal reflecting surfaces.

[0024] In the illustrated embodiments, the reflecting surfaces 72 are substantially planar, extend the length of the light assembly and are laterally equidistantly spaced. The reflecting surfaces 72 are oriented at an angle of approximately 45° relative to the optical axes A_O of the LEDs 20. The shape, height, length, lateral spacing, and angular orientation of the reflecting surfaces may be selected to produce a particular desired pattern of light distribution. For example, reflecting surfaces that are convex when viewed in section (not shown) would increase the spread of the light pattern for the assembly.

[0025] FIG. 5 illustrates the geometrical relationships between an LED 20 and the refracting and reflecting surfaces of one half 10a of the side-emitting collimator 10. Each half 10a, 10b of the side-emitting collimator 10 includes an upper surface 16 produced by rotating a non-circular curve 90 about an axis of symmetry A_C that extends beneath both halves 10a, 10b of the collimator 10. The shape of the curve 90 rotated about axis A_C is calculated to reflect light refracted by the corresponding refracting surface 12 into the desired path of the collimated beam. The path of light gathered by the refracting surface 12 in the collimator can be calculated from the known values of: the shape of the surface 12; the angle of light incident upon the surface 12; and the refractive properties of the collimator lens material (acrylic plastic)/air interface using Snell's Law.

[0026] These calculations produce a non-spherical, or aspheric reflecting surface 16. As used in this application, the term “aspheric” means “not spherical”. The illustrated aspheric surfaces 14, 16 are created by rotating a non-circular curve 90, 92 about an axis of symmetry A_C. The resulting aspheric surfaces can be described as “rotationally symmetric” about the axis of symmetry A_C. The non-circular curve 90 extends upwardly and outwardly from a first end 15 to a second end 17. When rotated at least approximately 180° about the axis of symmetry A_C, the curve 90 defines the upper aspheric reflecting surface 16 of each collimator half 10a, 10b.

[0027] The aspheric refracting surface 14 of each collimator half 10a, 10b is configured to refract a portion of the light from the LED 20 into a direction perpendicular to the optical axis A_O of the LED. The shape of the non-circular curve 92 used to define the aspheric refracting surface 14 is calculated from the known values of: the desired angle of refraction in the collimator; the angle of light incident upon the surface 14; and the refractive properties of the collimator lens material (acrylic plastic)/air interface using Snell's Law. The non-circular curve 92 extends between an origin 94 on the axis of symmetry A_C to a terminus 96 where the curve 92 intersects with the substantially cylindrical refracting surface 12. When rotated at least approximately 180° about the axis of symmetry A_C, the curve defines the aspheric refracting surface 14.

[0028] The illustrated collimator halves 10a, 10b are semi-circular in lateral section (perpendicular to the view of FIG. 5). A plane P, includes the axis of symmetry A_C and the focus F of the collimator 10. Each collimator half 10a, 10b, is rotationally symmetrical about the axis of symmetry A_C above plane P. Each half of the side-emitting collimator defines one half 30a, 30b of a cavity 30 that receives the light source (LED 20). The upper surface of the cavity includes two adjoining substantially cylindrical refracting surfaces 12 that meet over the optical axis A_O of the LED light source. Each end of the cavity 30 is defined by the aspheric refracting surface 14. The substantially cylindrical surfaces 12 are positioned to refract light from the light source onto the upper internal reflecting surface. The aspheric refracting surface 14 redirects a portion of the light from the light source into a direction parallel to light reflecting from the aspheric upper reflecting surface.

[0029] Each half 10a, 10b of the side-emitting collimator 10 defines one-half 30a, 30b of a cavity 30 surrounding the LED lens 28. The cavity portion 30a, 30b defined by each half 10a, 10b of the side-emitting collimator 10 is defined by two refracting surfaces 12, 14. The substantially cylindrical surface 12 is centered on focal point F and extends perpendicularly to the optical axis A_O of the LED 20. Surface 12 is described as a “substantially cylindrical” surface because it is not perfectly cylindrical with respect to the axis of symmetry A_C. In the illustrated embodiments, the substantially cylindrical surface 12 has a larger diameter at its open end (to the right in FIG. 5) than at the junction with the aspheric refracting surface 14. This particular surface configuration is not necessary to the optical performance of the collimator 10. Alternative surface configurations for the refracting surfaces may occur to one skilled in the art.

[0030] There are many ways to form the lens shapes employed in the side-emitting collimator 10. One preferred method is to mold the lenses from clear plastic. FIG. 6 illustrates an exemplary configuration where a row 40 of side-emitting collimator halves 10a are molded from optically clear acrylic plastic having an index of refraction of approximately 1.491 at a wavelength of 550 nm. A mirror image row 40 of side-emitting collimator halves arranged as shown in FIG. 9 produces eight side-emitting collimators 10 for organizing the light from a row of eight LEDs 20. Alternatively, the side-emitting collimators may be molded as a single unit (not shown).

[0031] FIGS. 7–10 illustrate exemplary light assemblies incorporating rows of side-emitting collimators 10 over corresponding rows of LEDs 20 according to aspects of the present invention. A molded plastic reflector 70 provides structural support for the assemblies. PC boards 50 include a linear array of eight LED's 20. Two rows 40 of side-emitting collimator halves 10a, 10b are arranged with their respective cavity ends aligned over the row of LEDs to form side-emitting collimators 10 over each LED. The PC boards and rows of collimator-halves are clamped against the back of the reflector 70 by heat sinks 60 corresponding to the configuration of the PC boards 50. The illustrated heat sinks 60 are molded from heat-transmissive plastic to disperse heat generated by the LED's 20. A lens 80 protects the assembly and also may be provided with refracting features for spreading light coming off the parallel reflecting surfaces 72.

[0032] FIG. 8 illustrates a light assembly employing a single PC board 50, heat sink 60 and row of side-emitting collimators 10 constructed from two rows 40 of collimator halves 10a, 10b. The heat sink 60, reflector 70 and lens 80 of FIG. 8 function in the same manner as the corresponding components of FIG. 7. FIG. 3 illustrates the relative positions of the LED 20, side-emitting collimator 10 and reflecting surfaces 72 of the light assembly shown in FIG. 8. These relationships organize light from the row of LED's 20 so that it is emitted substantially evenly over the area of the light assembly. The inventive side-emitting collimator 10 and reflector efficiently distributes the light from the LEDs to improve the aesthetic appearance of the light radiation pattern without impairing compliance with relevant standards for warning and signal light pattern and intensity.

[0033] FIG. 9 is a top perspective view of the reflector of FIG. 7 in functional conjunction with the rows 40 of collimator halves 10a, 10b. The opposed collimated beams from each side-emitting collimator 10 are incident upon five parallel reflecting surfaces 72 extending along either side of each row of LEDs. This configuration effectively spreads the light over the surface area of the reflector 70. FIG. 10 is a side view of the assembled components shown in FIG. 7. An advantage of the illustrated light assembly configuration is an extremely low profile as shown in FIG. 10. This low profile permits a light assembly according to the present invention to be mounted on the exterior of a vehicle without requiring a large cutout to accommodate a deep reflector. The rear surface of the heat sinks 60 will have a large surface contact with what will typically be the metal skin of the emergency vehicle. This large surface contact will enhance heat transfer away from the LEDs of the light assembly.

[0034] While exemplary embodiments of the foregoing invention have been set forth for purposes of illustration, the foregoing description should not be deemed a limitation of the invention herein. Accordingly, various modifications, adaptations and alternatives may occur to one of skill in the art without departing from the spirit and the scope of the present invention.